The laser fusion of thermoplastics is a novel method of high economic interest to manufacture plastics articles having complex geometry. In this method, the parts to be bonded are initially fixed in the correct end position relative to one another and the joining surfaces are brought into contact with one another. The bonding region is then irradiated with laser light through one of the parts until there is surface melting at least on the part lying below it and thus formation of a fusion bond between the parts which solidifies mechanically in the subsequent cooling phase.
For the absorption of the laser radiation and its conversion to heat for the melting operation, it is necessary to use absorbing materials which are either incorporated into at least one of the plastics parts or are applied thereto in such a way that they are disposed in the bonding region between the plastics parts.
Typically, for example also in EP-A-159 169, carbon black is used as the material which absorbs IR laser radiation. However, owing to the intrinsic color of the carbon black, this leads, if not to black coloration, to nontransparency of the plastics part or of the bonding region.
In addition to carbon black, DE-A-44 32 081 also recommends nigrosine as an IR-absorbing material for the fusion bonding of plastics parts. Nigrosine does not lead to discoloration of the plastic, but, owing to it having only low absorption in the NIR, comparatively high laser outputs are required at the wavelengths of 940 and 1064 nm used and lead readily to carbonization of the plastic. Moreover, nigrosine is toxic and carcinogenic.
EP-A-126 787 and DE-A-198 14 298 disclose the use of silicon dioxide as an absorber for the fusion of polyethylene films or the production of a fuel tank by fusing two hollow plastic bodies. However, silicon dioxide too does not exhibit satisfactory absorption at the laser wavelengths of 808, 940 and 1064 nm used.
Finally, WO-A-00/20157 and 03/59619 describe a process for laser-fusing plastics parts, in which dyes which substantially do not absorb in the visible region are used as an IR-absorbent coating of one plastics part. However, the dyes used do not have adequate photostability or radiation resistance. WO-A-03/59619 even explicitly mentions the decomposition of the dye. Accordingly, it is possible neither to employ multipass fusion nor quasisimultaneous fusion, in which the bridging of the gap is better and fewer stress cracks form, and only one-pass contour fusion is available. Moreover, the thermal stability of the dyes mentioned is inadequate for direct incorporation into the plastics parts to be fused by means of conventional melting methods such as extrusion, coextrusion, blow molding or injection molding. The dyes therefore have to be applied to one of the plastics parts in an additional working step, which is cost- and labor-intensive and unsuitable especially for mass production of parts which are small or have a complex shape. In addition, the dyes used and especially their photochemical and thermal degradation products are not chemically inert and tend to undesired migration in the matrix polymer.
It is accordingly an object of the present invention to remedy the disadvantages mentioned and to provide a universal process by which the plastics parts can be laser-fused in a simple manner from a process technology point of view.